Performance of Arrays and Hashes in Ruby

Question

I have a program that will store many instances of one class, let\'s say up to 10.000 or more. The class instances have several properties that I need from time to time, but

Answer 1

When using unique values, you can use the Ruby Set which has been previously mentioned. Here are benchmark results. It's slightly slower than the hash though.

                 user     system      total        real
array        0.460000   0.000000   0.460000 (  0.460666)
hash         0.000000   0.000000   0.000000 (  0.000219)
set          0.000000   0.000000   0.000000 (  0.000273)

I simply added to @steenslag's code which can be found here https://gist.github.com/rsiddle/a87df54191b6b9dfe7c9.

I used ruby 2.1.1p76 for this test.

Answer 2

Ruby has a set class in its standard library, have you considering keeping an (additional) set of IDs only?

http://stdlib.rubyonrails.org/libdoc/set/rdoc/index.html

To quote the docs: "This is a hybrid of Array’s intuitive inter-operation facilities and Hash’s fast lookup".

Answer 3

1. Use a Set of Documents. It has most of the properties you want (constant-time lookup and does not allow duplicates),. Smalltalkers would tell you that using a collection that already has the properties you want is most of the battle.

2. Use a Hash of Documents by document id, with ||= for conditional insertion (rather than has_key?).

Hashes are designed for constant-time insertion and lookup. Ruby's Set uses a Hash internally.

Be aware that your Document objects will need to implement #hash and #eql? properly in order for them to behave as you would expect as Hash keys or members of a set, as these are used to define hash equality.

Answer 4

Hashes are much faster for lookups:

require 'benchmark'
Document = Struct.new(:id,:a,:b,:c)
documents_a = []
documents_h = {}
1.upto(10_000) do |n|
  d = Document.new(n)
  documents_a << d
  documents_h[d.id] = d
end
searchlist = Array.new(1000){ rand(10_000)+1 }

Benchmark.bm(10) do |x|
  x.report('array'){searchlist.each{|el| documents_a.any?{|d| d.id == el}} }
  x.report('hash'){searchlist.each{|el| documents_h.has_key?(el)} }
end

#                user     system      total        real
#array       2.240000   0.020000   2.260000 (  2.370452)
#hash        0.000000   0.000000   0.000000 (  0.000695)